1. Field of the Invention
The present invention relates to an image coding device, which is capable of coding motion image signals without affecting the frame rate thereof when output at the decoding side, and also relates to a method for coding motion image signals.
2. Description of the Related Art
When digitizing motion image signals, the resulting data amount often becomes enormous, and thus in the case for storing motion image signals in a recording medium such as an optical disk, or the case for broadcasting and/or communicating these motion image signals, a coding operation is applied to those motion image signals for the purpose of reducing the total data amount thereof.
Many coding methods for motion image signals have been proposed so far, and in the case of the MPEG method (Motion Picture Experts Group) for example, which is standardized in the international level by the ISO/IEC, a hybrid coding method composed by a combination of the DCT (Discrete Cosine Transform) coding and the inter-frame differential coding with MC (Motion Compensating Prediction) is employed.
In the inter-frame differential coding with the motion compensating prediction, a motion vector is calculated from a coded image and an image to be coded per blocks of a predetermined size, and the differential signal obtained from the both images and the motion vector are coded.
In the discrete cosine transform coding, the amount of data after coding (or coded amount) can be reduced by transforming an image to be coded or the above-mentioned differential signal from the spatial area to the frequency area, and quantizing the thus generated DCT coefficients in accordance with the step size in quantization in compliance with the transmission rate. This quantization step size is set to either one of preset areas (namely an integer number from 1 to 31 in the case of the MPEG method, for example), wherein if the quantization step size is made large, the total amount of coded data is greatly reduced, although the resultant image quality is deteriorated.
For example, in the digital broadcasting case, if the MPEG method is employed, the amount of coded data is normally compressed from 1/20 to 1/50. However, it is also well known that if the compression rate is to be made still higher, various noises in coding such as “block noise”, “mosquito noise” and so on, which are perceptible to the human's ears, are generated, so that the resultant quality of the coded images is drastically deteriorated.
Due to these facts, in the case where higher rate of data compression is to be obtained, yet suppressing generation of the noise in coding at the same time, it has conventionally been completed out by skipping some frames in accordance with the characteristic of input motion image signals, using the motion image coding method as disclosed in Japanese Patent Application Laid Open No. 9-168155. In this case, if, for example, the H.261 method standardized by the ITU-T is employed for the coding operation, the coded data is decoded just as it is at the decoding side, and as for the image of the skipped frames, image of a frame preceding each of the excluded frames is displayed in a continued mode.
However, since the conventional image coding method is configured as explained above, in the coding system like the MPEG 2 method in which the frame rate of a motion image signal is set to a constant level when output at its decoding time, it cannot be arranged such that the coded data is decoded as it is at the decoding side, and the image of the frames preceding each of the skipped frames is displayed in succession for the image of the skipped frames. Due to this, there has been still a problem that when a coding system such as the MPEG 2 is employed in which it is prescribed such that the frame rate of a motion image signal is set to a constant level when output at its decoding time, a data compression with minimum deterioration of image quality, yet without affecting the frame rate output at the decoding side cannot be realized.
The present invention has been proposed to solve the problem aforementioned, and it is an object of the present invention to provide an image coding device, which is capable of carrying out a data compression with minimum deterioration of image quality, yet without affecting the frame rate output at the decoding side by controlling in advance the frame rate of an input image signal and/or image size as a preprocess, coding thereafter the thus controlled motion image signal on the basis of the preprocess using the coding system in which it is prescribed such that the frame rate of a motion image signal is set to a constant level when output at its decoding time, and outputting the thus coded data in conformity with the used coding method. It is also another object of the present invention to provide a method of image coding device.